Recently, according to the trend of display devices becoming lighter and smaller, a low energy consuming LCD is being actively developed. The most widely used LCD until now are either the simple matrix type super twisted nematic (STN) LCD using nematic liquid crystal which is passively driven, or the active matrix type twisted nematic (TN) LCD which is driven by semiconductor device. However, these have a slow response time and limitation in displaying images of a large capacity and high density.
In 1971, ferroelectric liquid crystal was first obtained which had a chiral carbon in its molecular structure and exhibited a chiral smectic phase (Sc*). Owing to spontaneous polarization due to a vertical polar moment acting on the surface of the molecules, ferroelectric liquid crystal has a very fast response time and bistability giving its good memory characteristics. Accordingly, a display of virtually any size can be made by using a simple driving method.
Since the spontaneous polarity of the ferroelectric liquid crystal dissipates in a bulk state, appropriate surface treatment is carried out when it is applied to a display.
The concept of a surface stabilized ferroelectric LCD can prevent the dissipation of the spontaneous polarity. To obtain surface stabilization, the thickness of liquid crystal cell should be adjusted to 1.5.about.2 .mu.m and the surface interaction energy of the liquid crystal is appropriately adjusted by coating and rubbing a polymeric orientating material on the transparent electrode.
When external voltage is applied to the liquid crystal molecules having the spontaneous polarity, the polarity tends to shift to the direction parallel to the electrical field. Accordingly, an electric charge is induced on both sides of a liquid crystal layer, so that an internal electrical field (i.e., an anti-electrical field) is generated in the reverse direction of the external electrical field. The anti-electrical field plays the role of changing the direction of the spontaneous polarization to the original direction. This significantly deteriorates the memory characteristics of the liquid crystal and hence degrades the contrast ratio of the display device. However, since the spontaneous polarity is followed by the generation of the anti-electrical field, a fast removal thereof is required.
An orientation layer formed by coating common polymeric orientating material, including the widely used polyimide, is too thick and its insulating effect is too large. Accordingly, the anti-electrical field remains for too long of a time period when compared with the reversing time of the spontaneous polarity. Recently, methods using ultra thin films such as LB for the orientation layer, mixing charge transferring complexes with the liquid crystal mixture, and adding conductive polymer to the orientating material, etc. have been suggested to solve the above problem. Among these methods, it is preferable to use a material having a charge on the surface since the anti-electrical field formed when electrical field is applied thereto dissipates rapidly and the phenomena of memory characteristic deterioration can be prevented.